192 research outputs found
Phase behavior of a system of particles with core collapse
The pressure-temperature phase diagram of a one-component system, with
particles interacting through a spherically symmetric pair potential in two
dimensions is studied. The interaction consists of a hard core plus an
additional repulsion at low energies. It is shown that at zero temperature,
instead of the expected isostructural transition due to core collapse occurring
when increasing pressure, the system passes through a series of ground states
that are not triangular lattices. In particular, and depending on parameters,
structures with squares, chains, hexagons and even quasicrystalline ground
states are found. At finite temperatures the solid-fluid coexistence line
presents a zone with negative slope (which implies melting with decreasing in
volume) and the fluid phase has a temperature of maximum density, similar to
that in water.Comment: 11 pages, 15 figures included. To appear in PRE. Some figures in low
quality format. Better ones available upon request from [email protected]
Phase Transitions of Hard Disks in External Periodic Potentials: A Monte Carlo Study
The nature of freezing and melting transitions for a system of hard disks in
a spatially periodic external potential is studied using extensive Monte Carlo
simulations. Detailed finite size scaling analysis of various thermodynamic
quantities like the order parameter, its cumulants etc. are used to map the
phase diagram of the system for various values of the density and the amplitude
of the external potential. We find clear indication of a re-entrant liquid
phase over a significant region of the parameter space. Our simulations
therefore show that the system of hard disks behaves in a fashion similar to
charge stabilized colloids which are known to undergo an initial freezing,
followed by a re-melting transition as the amplitude of the imposed, modulating
field produced by crossed laser beams is steadily increased. Detailed analysis
of our data shows several features consistent with a recent dislocation
unbinding theory of laser induced melting.Comment: 36 pages, 16 figure
Phase Transitions of Soft Disks in External Periodic Potentials: A Monte Carlo Study
The nature of freezing and melting transitions for a system of model colloids
interacting by a DLVO potential in a spatially periodic external potential is
studied using extensive Monte Carlo simulations. Detailed finite size scaling
analyses of various thermodynamic quantities like the order parameter, its
cumulants etc. are used to map the phase diagram of the system for various
values of the reduced screening length and the amplitude of the
external potential. We find clear indication of a reentrant liquid phase over a
significant region of the parameter space. Our simulations therefore show that
the system of soft disks behaves in a fashion similar to charge stabilized
colloids which are known to undergo an initial freezing, followed by a
re-melting transition as the amplitude of the imposed, modulating field
produced by crossed laser beams is steadily increased. Detailed analysis of our
data shows several features consistent with a recent dislocation unbinding
theory of laser induced melting
Two-dimensional XY spin/gauge glasses on periodic and quasiperiodic lattices
Via Monte Carlo studies of the frustrated XY or classical planar model we
demonstrate the possibility of a finite (nonzero) temperature spin/gauge glass
phase in two dimensions. Examples of both periodic and quasiperiodic two
dimensional lattices, where a high temperature paramagnetic phase changes to a
spin/gauge glass phase with the lowering of temperature, are presented. The
existence of the spin/gauge glass phase is substantiated by our study of the
temperature dependence of the Edwards-Anderson order parameter, spin glass
susceptibility, linear susceptibility and the specific heat. Finite size
scaling analysis of spin glass susceptibility and order parameter yields a
nonzero critical temperature and exponents that are in close agreement with
those obtained by Bhatt and Young in their random Ising model study
on a square lattice. These results suggest that certain periodic and
quasiperiodic two-dimensional arrays of superconducting grains in suitably
chosen transverse magnetic fields should behave as superconducting glasses at
low temperatures.Comment: RevTex, 25 pages. 11 epsf figures available upon request
([email protected] or [email protected]). Submitted
to Phys. Rev.
Depinning Transition of a Two Dimensional Vortex Lattice in a Commensurate Periodic Potential
We use Monte Carlo simulations of the 2D one component Coulomb gas on a
triangular lattice, to study the depinning transition of a 2D vortex lattice in
a commensurate periodic potential. A detailed finite size scaling analysis
indicates this transition to be first order. No significant changes in behavior
were found as vortex density was varied over a wide range.Comment: 5 pages, 8 figures. Revised discussion of correlation length exponent
using a more accurate finite size scaling analysis. New figs. 5 and 6. Old
figs. 6 and 7 now figs. 7 and
Spin glass behavior of frustrated 2-D Penrose lattice in the classical planar model
Via extensive Monte Carlo studies we show that the frustrated XY Hamiltonian
on a 2-D Penrose lattice admits of a spin glass phase at low temperature.
Studies of the Edwards-Anderson order parameter, spin glass susceptibility, and
local (linear) susceptibility point unequivocally to a paramagnetic to spin
glass transition as the temperature is lowered. Specific heat shows a rounded
peak at a temperature above the spin glass transition temperature, as is
commonly observed in spin glasses. Our results strongly suggest that the
critical point exponents are the same as obtained by Bhatt and Young in the
Ising model on a square lattice. However, unlike in the latter case,
the critical temperature is clearly finite (nonzero). The results imply that a
quasiperiodic 2-D array of superconducting grains in a suitably chosen
transverse magnetic field should behave as a superconducting glass at low
temperature.Comment: RevTex, 4 pages Including 4 figures. To appear in the June 1 1996
issue of Phys. Rev. B (Rapid Communications). Revised/replaced edition
contains an erratum at the end of the paper, also to appear in Phys. Rev.
Prediction of Emerging Technologies Based on Analysis of the U.S. Patent Citation Network
The network of patents connected by citations is an evolving graph, which
provides a representation of the innovation process. A patent citing another
implies that the cited patent reflects a piece of previously existing knowledge
that the citing patent builds upon. A methodology presented here (i) identifies
actual clusters of patents: i.e. technological branches, and (ii) gives
predictions about the temporal changes of the structure of the clusters. A
predictor, called the {citation vector}, is defined for characterizing
technological development to show how a patent cited by other patents belongs
to various industrial fields. The clustering technique adopted is able to
detect the new emerging recombinations, and predicts emerging new technology
clusters. The predictive ability of our new method is illustrated on the
example of USPTO subcategory 11, Agriculture, Food, Textiles. A cluster of
patents is determined based on citation data up to 1991, which shows
significant overlap of the class 442 formed at the beginning of 1997. These new
tools of predictive analytics could support policy decision making processes in
science and technology, and help formulate recommendations for action
Van der Waals loops and the melting transition in two dimensions
Evidence for the existence of van der Waals loops in pressure p versus volume
v plots has for some time supported the belief that melting in two dimensions
is a first order phase transition. We report rather accurate equilibrium p(v)
curves for systems of hard disks obtained from long Monte Carlo simulations.
These curves, obtained in the constant volume ensemble, using periodic boundary
conditions, exhibit well defined van der Waals loops. We illustrate their
existence for finite systems that are known to undergo a continuous transition
in the thermodynamic limit. To this end, we obtain magnetization m versus
applied field curves from Monte Carlo simulations of the 2D Ising model, in the
constant m ensemble, at the critical point. Whether van der Waals loops for
disk systems behave in the thermodynamic limit as they do for the 2D Ising
model at the critical point cannot be ruled out. Thus, the often made claim
that melting in 2D is a first order phase transition, based on the evidence
that van der Waals loops exist, is not sound.Comment: 10 pages, 6 Postscript figures (submitted to Phys.Rev.E). For related
work, see http://pipe.unizar.es/~jf
Phase Transitions Driven by Vortices in 2D Superfluids and Superconductors: From Kosterlitz-Thouless to 1st Order
The Landau-Ginzburg-Wilson hamiltonian is studied for different values of the
parameter which multiplies the quartic term (it turns out that this
is equivalent to consider different values of the coherence length in
units of the lattice spacing ). It is observed that amplitude fluctuations
can change dramatically the nature of the phase transition: for small values of
(), instead of the smooth Kosterlitz-Thouless transition
there is a {\em first order} transition with a discontinuous jump in the vortex
density and a larger non-universal drop in the helicity modulus. In
particular, for sufficiently small (), the density of
bound pairs of vortex-antivortex below is so low that, drops to zero
almost for all temperature .Comment: 8 pages, 5 .eps figure
Phason elasticity of a three-dimensional quasicrystal: transfer-matrix method
We introduce a new transfer matrix method for calculating the thermodynamic
properties of random-tiling models of quasicrystals in any number of
dimensions, and describe how it may be used to calculate the phason elastic
properties of these models, which are related to experimental measurables such
as phason Debye-Waller factors, and diffuse scattering wings near Bragg peaks.
We apply our method to the canonical-cell model of the icosahedral phase,
making use of results from a previously-presented calculation in which the
possible structures for this model under specific periodic boundary conditions
were cataloged using a computational technique. We give results for the
configurational entropy density and the two fundamental elastic constants for a
range of system sizes. The method is general enough allow a similar calculation
to be performed for any other random tiling model.Comment: 38 pages, 3 PostScript figures, self-expanding uuencoded compressed
tar file, LaTeX using RevTeX macros and epsfig.st
- …